'use strict';

function main() {
  // Get A WebGL context
  var canvas = document.querySelector('#c');
  var gl = canvas.getContext('webgl');
  if (!gl) {
    return;
  }

  // Use our boilerplate utils to compile the shaders and link into a program
  var program = webglUtils.createProgramFromScripts(gl, [
    'vertex-shader-2d',
    'fragment-shader-2d',
  ]);

  // look up where the vertex data needs to go.
  var positionAttributeLocation = gl.getAttribLocation(program, 'a_position');

  // 找到设置的变量u_resolution 分辨率
  var resolutionUniformLocation = gl.getUniformLocation(
    program,
    'u_resolution'
  );

  // Create a buffer to put three 2d clip space points in
  var positionBuffer = gl.createBuffer();

  // Bind it to ARRAY_BUFFER (think of it as ARRAY_BUFFER = positionBuffer)
  gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);

  var positions = [10, 20, 80, 20, 10, 30, 10, 30, 80, 20, 80, 30];
  gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(positions), gl.STATIC_DRAW);

  webglUtils.resizeCanvasToDisplaySize(gl.canvas);

  // Tell WebGL how to convert from clip space to pixels
  gl.viewport(0, 0, gl.canvas.width, gl.canvas.height);

  // Clear the canvas
  gl.clearColor(0, 0, 0, 0);
  gl.clear(gl.COLOR_BUFFER_BIT);

  // Tell it to use our program (pair of shaders)
  gl.useProgram(program);

  // Turn on the attribute
  gl.enableVertexAttribArray(positionAttributeLocation);

  // Bind the position buffer.
  gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);

  // Tell the attribute how to get data out of positionBuffer (ARRAY_BUFFER)
  var size = 2; // 2 components per iteration
  var type = gl.FLOAT; // the data is 32bit floats
  var normalize = false; // don't normalize the data
  var stride = 0; // 0 = move forward size * sizeof(type) each iteration to get the next position
  var offset = 0; // start at the beginning of the buffer
  gl.vertexAttribPointer(
    positionAttributeLocation,
    size,
    type,
    normalize,
    stride,
    offset
  );

  // ------设置全局变量 分辨率
  gl.uniform2f(resolutionUniformLocation, gl.canvas.width, gl.canvas.height);

  // draw
  var primitiveType = gl.TRIANGLES;
  var offset = 0;
  var count = 6;
  gl.drawArrays(primitiveType, offset, count);
}

main();
